Before commencing excavation or other work where electrical cables, fibre optic cables or other utilities ducts or pipes are buried, it is important to determine the location of such buried cables or pipes to ensure that they are not damaged during the work. Once a buried utility is located the depth of the utility can be calculated to determine a safe excavation depth.
Current carrying conductors emit electromagnetic radiation which can be detected by an electrical antenna. If fibre optic cables or non-metallic utilities ducts or pipes are fitted with a small electrical tracer line, an alternating electrical current can be induced in the tracer line which in turn radiates electromagnetic radiation. It is known to use detectors to detect the electromagnetic field emitted by conductors carrying alternating current.
One type of such detector works in one of two modes, namely ‘active’ or ‘passive’ modes. Each mode has its own frequency bands of detection.
The passive mode comprises ‘power’ mode and ‘radio’ mode. In power mode, the detector detects the magnetic field produced by a conductor carrying an AC mains power supply at 50/60 Hz, or the magnetic field re-radiated from a conductor as a result of a nearby cable carrying AC power, together with higher harmonics up to about 5 KHz. In radio mode, the detector detects very low frequency (VLF) radio energy which is re-radiated by buried conductors. The source of the original VLF radio signals is a plurality of VLF long wave transmitters, both commercial and military.
In the active mode, a signal transmitter produces an alternating magnetic field of known frequency and modulation, which induces a current in a nearby buried conductor. The signal transmitter may be directly connected to the conductor or, where direct connection access is not possible, a signal transmitter may be placed near to the buried conductor and a signal may be induced in the conductor. The buried conductor re-radiates the signal produced by the signal transmitter.